This invention generally relates to the manufacture of paperboard products. In particular, the invention relates to multi-ply paperboard products having low score-line cracking propensity.
There have been available few tests for evaluating score-line cracking, for instance, the score cracking angle test disclosed by Whitsitt and McKee in xe2x80x9cInvestigation of Improved Device for Evaluating the Cracking Potential of Linerboard,xe2x80x9d Institute of Paper Chemistry Summary Report, Project No. 1108-29 (1996). This test, first developed at the Institute of Paper Chemistry, fails to either measure a fundamental material property, or detect damage in a single ply. No meaningful correlations have thus been found over the years to score-line cracking performance in the field.
A recent test, disclosed by J. Gonzalez in xe2x80x9cScore Cracking in Linerboard,xe2x80x9d M.S. Dissertation (No. 6190-Research), Institute of Paper Science and Technology, Georgia (2000), is not so much a predictive test, but rather is a set-up that attempts to replicate, rather poorly, scoring. It does not scientifically measure any board property that may prove to correlate to score-line cracking propensity in the field. The test comprises two motor-driven horizontal metal wheels forming essentially a nip compression. A linerboard sample (25 cmxc3x9712 cm), manually folded in half and fed between the flat metal wheels, undergoes a nip-type compressive force. The cracking percent of the folded sample is then measured visually or using a microscope.
The addition of starch to linerboard contributes to embrittling the board, hence reducing its ability to deform plastically, thereby speeding up catastrophic failure. Machine trials and previous experimental work conducted by the inventors confirmed the embrittling nature of (increasing) starch content in linerboard.
Experimental work has provided insight into the root causes of the propensity for score-line cracking in white-top linerboard (especially 69 lb.). [The designation xe2x80x9cwhite-topxe2x80x9d refers herein to linerboard products which contain more than one layer of fibers with the top layer being composed of recycled or virgin bleached pulp.] The mechanistic basis for designing a score-line-crack-resistant two-ply linerboard is grounded in three (non-mutually exclusive) functional factors: (1) the ability of the top ply to undergo large plastic (irreversible) deformation prior to failure; (2) the ability of the base ply to compress elastically while the top ply is deforming (plastically); and (3) in order for (1) and (2) to simultaneously apply, the interlaminar (ply) bond must be low enough (but adequate to ensure against delamination) to allow the top ply to xe2x80x9cslidexe2x80x9d over the base ply. Achieving this requires a testing method capable of predicting plastic deformations, or the fracture toughness, of the top ply alone, and able to correlate such a measurement with score-line cracking propensity in the field.
In materials science and engineering, the term xe2x80x9cmaterialxe2x80x9d has a precise meaning. It refers to either a pure substance or an alloy that can be approximated as essentially homogeneous in composition. When more than one substance or material are combined, and when this combination has internal structural heterogeneity, the term xe2x80x9ccomposite materialxe2x80x9d is used. According to this definition, wood fibers may be regarded as composite materials, or, specifically, composite tubes of cellulosic microfibrils embedded in an amorphous matrix of hemicellulose and lignin. Structurally, paper or board is, however, a network. On a microscopic scale, paper or board is a cellulosic network of crossing fibers filled with voids; macroscopically, it could be regarded as a continuum with inherent (micro)cracks and flaws being xe2x80x9csmeared outxe2x80x9d for the purpose of analysis. For practical issues related, for instance, to box construction, such as scoring, it may be deemed appropriate that linerboard be dealt with as a continuum whose material properties and structural analysis are determined relying on theories of elasticity and plasticity from the field of solid mechanics. Thus, two-ply linerboard constructs comprise two elastic-plastic sheet-like materials whose properties may be analyzed orthotropically. Safeguarding against, for instance, cracking in the top ply during scoring would necessitate attention principally to: i) the extent of (plastic) deformability in each ply; and ii) inter-ply stresses.
Linear elastic materials load and unload along the same path; crack growth in such a material can be represented graphically by a load-displacement curve which is linear up to the point of crack propagation, and the displacement is zero when the specimen is unloaded. The energy consumed in the fracture process is therefore equivalent to the area enclosed under the curve. The irreversible work consumed during elastic fracture is confined to thin boundary layers along the faces of the propagating crack.
Paper and board, however, are tough, ductile materials (the extent of which depends on furnish composition and papermaking conditions) whose yield stress is low. When such a material is strained, it yields not only at the point(s) of crack initiation, but away from these points too. Thus, irreversible deformation is no longer confined to the thin boundary layer along the faces of the propagated crack (as in elastic fracture), but is spread throughout the material. In addition to the work required in the crack tip process zone, significant irreversible work is consumed in the yielded regions away from the crack. It is important to recognize that the plastic deformation outside the fracture process zone is not essential to the process of fracture. Consequences of the plastic flow include curvature in the load-displacement curve on loading, and displacement irreversibilities upon unloading, both in a specimen without a crack and a specimen with a crack.
The work done during loading, given by the area under the load-displacement curve, represents the combined contribution to fracture and remote flow. These two works are difficult to separate experimentally. However, a methodology has been developed to separate the elastic and plastic portions of the fracture energy consumed in deforming the top-ply of two-ply linerboard systems. This methodology has been designed so that the measured plastic contribution of the work done during the fracture process correlates well with predicting the propensity of linerboard to score-line cracking during converting operations.
More specifically, a top-ply fracture tester has been developed which comprises two clamps in which a sample is placed; one of the clamps is fixed, the other rotates the sample around a spindle. When bending the sample, it is under a net tensile force, which is recorded using a load cell. The testing set-up measures the fracture energy (and toughness) in a single ply of a multi-ply structure. The top-ply fracture tester induces fracture in the top ply only, and allows the identification of elastic and plastic regions in a single ply. Score-line cracking resistance essentially emanates from the ability of the sheet to deform plastically (in the top ply), and analysis has shown that it is (perfectly) correlated with the two components of the top-ply plastic zone: the work done, or energy consumed, during the crack-to-gap transition and during the gap-to-flap transition. This accurate measure formed the cornerstone for experimentation into examining the root cause(s) of score-line cracking, and paved the path for the ultimate design of crack-resistant linerboard.
The present invention is directed to a method for engineering a multi-ply paperboard system having enhanced resistance to score-line cracking. The research work conclusively indicated that the propensity for score-line cracking may be eliminated by engineering a ductile, plastically-deforming top ply of the multi-ply construction. Plasticity essentially extends the life span through which a material may deform (under external monotonic loading) prior to the initiation of damage (e.g., crack propagation) and ultimate failure. Hence the board""s propensity for cracking is intrinsically linked to its fracture energy, or the energy consumed during plastic deformation (prior to failure).
In accordance with one preferred embodiment, the methodology of statistical design of experiments (DOE) is employed to engineer a score-line-crack-resistant white-top linerboard. A statistically designed experiment allows one to efficiently make empirical measurements at the limits of the parameter ranges, and with minimal number of experiments or xe2x80x9ctrialsxe2x80x9d, derive a prediction equation of state which describes the process of manufacturing crack-resistant linerboard. First, a list of the principal factors in the manufacture of score-line-crack-resistant linerboard was derived based on experimental findings. In accordance with the preferred embodiment, the principal variables that have been identified to affect the propensity for score-line cracking are: fiber furnish (virgin vs. recycled), top-ply coverage (i.e., weight of the top ply), refining energy of the top-ply furnish, and the starch and filler contents. Pulp quality, especially pulp viscosity, was also shown to play an important role in influencing cracking resistance.
Selected input factors were varied in a statistically designed experiment. The response variables were then physically measured. In particular, a specially designed top-ply fracture tester was used to measure the plastic energy consumed in deforming a single ply in the multi-ply system. The energy consumed in the plastic zone can be characterized by two components related to the level of damage in the top ply, viz.: crack, the point of crack initiation; gap, the further propagation of the cracks; and flap, the consolidation of the cracks into rupture zones. The two components, energy consumed during the crack-to-gap transition and energy consumed during the gap-to-flap transition, have shown excellent correlations to score-line cracking field performance of white-top linerboard.
Following the measurement of the physical properties of interest, a statistical analysis of the response variables as a function of the input (DOE) variables was used to derive predictive equations of state. The result of these experimental studies was a set of proposed values for key operational parameters for the production of white-top linerboard having optimal score-line cracking resistance.
The invention is further directed to a method of manufacturing score-line-crack-resistant linerboard in which key operational values are optimized.